Breakwater generating structure

ABSTRACT

Waves are amplified by the gradual decrease of the depth, and when the waves reach to a vertical wall  10  of a reef  2 , the breaking waves are generated by sudden decrease of the water depth. As the breaking waves rush into the upper portion of the reef  2  and pass through the slant slits  14 , and wave energy are absorbed by the reef  2  and the seat water returns back to the ocean through an opening  11 . The sand brought into a reef  2  with the waves are washed away by the return flow  11  so that the sand is not deposited within the reef  2  and the space in the reef  2  is always clear.  
     The return flow promotes generation of the breaking waves. The breaking waves are introduced into the slit, the beach erosion is prevented, and also the calm sea area utilized for the marine leisure is created. The seawater in the reef with sufficient air flows to the sea area behind the breakwater through the paths  19 . The sea water containing sufficient oxygen is supplied to the bottom layer behind the breakwater without disturbing the surface so that the oxygen is supplied to the seawater behind the breakwater.

FIELD OF THE INVENTION

[0001] The present invention relates to coastal erosion control ingeneral and relates specifically to a breakwater generating structuresuch as a submerged breakwater generating structure or an offshorebreakwater generating structure.

PRIOR ARTS

[0002] For beach erosion control or creation of a calm sea water areafor marine leisure, various breakwater generating structures using wavebreaking have been proposed.

[0003] Japanese Patent Laid-Open Hei 4-289310(1992-289610,A,JP)discloses a breakwater generating structure that uses terracedhorizontal plates of which the deepest part is placed with facing theoffshore side. Japanese Utility Model Laid-Open Hei4-57518(1992-57518,U,JP) discloses a structure which uses parallelinclined plates. Japanese Patent Laid-Open Hei4-136311(1992-136311,A,JP) discloses a structure wherein a submergedbreakwater is constructed at the offshore side of the main breakwatergenerating structure. In addition, as shown in FIG. 11, Japanese PatentLaid-Open Hei 10-2565(1998-2565,A,JP) discloses a structure usingcomposite wave breaking.

[0004] However, in the above submerged breakwaters or an artificialreef, the structures therefore tend to be inevitably huge, because ofthe breakwater principle in which the breakwater effect is dependent onthe shallowness of the breakwater generating structures and the width (awave propagation direction)

[0005] For example, in the above breakwater generating structuresutilizing composite breaking waves, special composite breaking waves aregenerated with the double reef structure comprising an upper reef and alower reef of which length is designed to be 6.5 times longer than thatof the depth of water.

[0006] In order to obtain sufficient breakwater effect, the structuremust be huge enough, thereby the construction cost is high and theconstruction period becomes long.

[0007] An objective of the present invention is to provide a breakwatergenerating structure such as a submerged breakwater generating structureby utilizing the composite wave breaking which is comparatively smalland is constructed with lower cost.

[0008] In addition, when the breakwater prevents the waves effectivelyfrom reaching the coast, a coastal area behind the submerged breakwatergenerating structure becomes calm, thereby gathering fish and othercreatures behind the breakwater (swarming effect). However, when seawater stays still at a bottom layer, sludge may be deposited and the seawater exchange inside the breakwater takes a long time, and meanwhile,bacteria may consume much dissolved oxygen in the sea water to decomposeorganic materials in the sludge and the dissolved oxyegen becomes low,thereby resulting in bad influence to lives such as those of fish andshellfish.

[0009] Thus, the invention also enables efficient exchange of seawaterbehind the breakwater for supplying sufficient dissolved oxygen in theseawater for the lives.

SUMMARY OF THE INVENTION

[0010] The breakwater generating structure of the present invention isconstructed compactly by providing an vertical wall at the offshore sideof the reef, forming the openings at the bottom end of the verticalwall, and further providing the slit protruding inside the reef on a topof the structure.

[0011] The slant slits are constructed by slit plates and the slitplates are inclined toward the direction along which the wavespropagate, thereby capturing the breaking waves in the reef.

[0012] Breaking waves rushed into the reef of the breakwater generatingstructure of the two-stage reef structure comprising the reefconstructed on the mound is guided to the coast side. By providing thethrough paths at the coast side of the reef which forms the upper reef,the sea water containing air brought by the breaking waves is sent tothe water area at the coast side to solve the lean oxygen state causedby stagnation of the seawater in the bottom layer behind the breakwater.

[0013] Wave amplitude is amplified by a slope of the sea bottom or thelower reef such as a mound as approaching the breakwater generatingstructure, and then greater breaking waves than usual are generated bysudden decrease of the water depth at the vertical wall portion of thereef. Furthermore, because these breaking waves rushing into the slitdecrease a conveyance rate of waves toward the coast, a calm sea area iscreated at the coast side thereof.

[0014] The breaking waves rushing into the reef create a return waterflow toward the offshore side through the opening of the vertical walland sand brought into the reef is discharged by and together with thereturn water flow so as to prevent accumulation of the sand in the reef.Furthermore, the return water flow helps to generate breaking waves atthe vertical wall portion while shifting the point of breaking wave.Because the breaking waves can easily rush into the slits so that a waveenergy may be lowered and a breakwater generating effect may beenhanced.

[0015] In addition, by providing a close portion defined by the upperportion continuous to the vertical wall together with the vertical wall,generation of breaking waves may be promoted by the return water flow onan upper surface of the closed portion toward the offshore side, at thesame time the breaking wave point is shifted so that the breaking wavesmay accurately rush into the slits.

[0016] The embodiment, in which the reef with the opening and the slitis placed on a mound, a lower reef,

[0017] shown in FIG. 1 to form a breakwater generating structure 1, isdescribed herein below.

[0018] An water depth where the breakwater generating structure 1 isplaced is h₁, the total length of a mound 3 on which a reef 2 is placedis L, and the height of said mound 3 is R₁. The reef 2 having the lengthX₂, the vertical wall with the height R₂, and the opening with theheight R₂ is placed on said mound so that the vertical wall 10 ispositioned at distance X, from the offshore end of the mound. The depthfrom the water surface to the top of the structure is R₃.

[0019] On the upper portion of the reef 2, the slant slits 14 aredisposed with spacing each other and are inclined to the angle (è) wherethe braking waves rush into the water surface with respect to thedirection of the wave propagation.

[0020] Preferably, the depth of the top of the reef R₃ is not more than1.5 m from the water level in a viewpoint of a breakwater generatingeffect, however, the depth should be selected by considering an effectto cruising ships. The depth of the top of the reef from the water levelmay be about 0.5 m when taking a measure in which buoys on the seaaround the breakwater generating structure are placed.

[0021] On the mound 3 of the reef 2 at the coast side, the ripraps orconcrete blocks are laid for a coast side lower reef length X₃ toprevent the reef 2 from being moved by waves.

[0022] Waves of height H₀ approaches the breakwater generatingstructure, the wave height is amplified as the depth becomes shallowerat the mound of the lower reef, and when the waves reach the verticalwall 10 of the reef 2, the breaking wave is generated because of suddendecrease of the depth. The breaking waves rush into the upper face ofthe reef 2 and pass through the slits 14, the wave energy is consumedand the return water flow toward the opening 11 is generated so thatsands brought into the reef may be discharged from the opening 11together with the return water flow. Thus the sands are not accumulatedin the reef and the space inside the reef is always maintained.

[0023] Creation of the return water flow from th r e opening 11 to theoffshore shifts the wave breaking point of the generated breaking wavestoward the offshore and also helps the breaking waves to rush into theslit 14 to lower the wave energy effectively.

[0024] The vertical blocks or ripraps on the coast side of the reef 2dissipate waves that are not captured by the slit 14 and also lower theenergy converted from the waves to the flow at the upper portion of thereef. Then the waves that have passed through the breakwater generatingstructure are attenuated to the wave height H₁.

[0025] As a result of an experiment, to create the calm sea area bydecreasing the ratio between the wave height H₁ and H₀ to not more than0.3, it was found that the following relations of parameters for thebreakwater generating structure were required:

[0026] Lower reef height: R₁=h₁/3 to h₁/2

[0027] Vertical wall height: R₂=h₁/3 to h₁/2

[0028] Opening height: R₄=R₂/10 to R₂/3

[0029] Lower reef length: X₁/1h₁ to 3h₁

[0030] Reef length: X₂=2h₁ to 4h₁

[0031] Coast side lower reef length: X₃=1h₁ to 3h₁

[0032] Slit panel angle: θ=25 to 45°

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a cross sectional view of a breakwater generatingstructure.

[0034]FIG. 2 is a cross sectional view of a breakwater generatingstructure.

[0035]FIG. 3 is a front view of a breakwater generating structure.

[0036]FIG. 4 is a sectional view of a breakwater generating structurehaving double reef structure.

[0037]FIG. 5 is a cross sectional view of a breakwater generatingstructure placed on a support structure.

[0038]FIG. 6 is a cross sectional view of a breakwater generatingstructure with a closed portion placed on an upper surface of a reef.

[0039]FIG. 7 is a front view of the breakwater generating structure withlegs.

[0040]FIG. 8 is a sectional view of an embodiment of a breakwatergenerating structure.

[0041]FIG. 9 is a front view from the offshore side and from the coastside of a breakwater generating structure.

[0042]FIG. 10 is a front sectional view of another embodiment of thebreakwater generating structure.

[0043]FIG. 11 is a sectional view of the conventional breakwatergenerating structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings.

[0045] Embodiments

[0046] As shown in FIGS. 2 and 3, a reef 2 of the breakwater generatingstructure 1 consists of a pre-stressed concrete box structure with awidth of 10 m, a height of 3 m and a length of 20 m. An upper portion ofthe box is opened. An end of the reef 2 facing to the offshore is avertical wall with an opening 11 through the lower end of the verticalwall at the height of 1 m. The inner space of the reef 2 is partitionedby partition walls 12. On the upper portion of the reef 2, the slitplates 13 are placed with spacing each other between the partition walls12 and the slit plates 13 are inclined with respect to the directionalong which waves propagate to form the slant slits 14. The reef 2comprises one unit, and the units are placed along with the coast linefor required numbers of units.

[0047] The reef 2 is placed on the slope of the sea bottom aslantgradually from the coast so that the top of the reef becomes to be 1.5 mfrom the water surface. The ripraps are laid at the coast side of thereef 2 to prevent the reef 2 from being moved by waves.

[0048] Waves propagating from the offshore are amplified as the waterdepth decrease, and when the waves reach the vertical wall 10 of thereef 2, breaking waves are created by sudden decrease of the waterdepth. The breaking waves rush over the reef 2 and pass through theslant slit 14 so that the energy thereof may be decreased inside thereef and the return water flow toward the opening 11 is generated. Thesand brought into the reef may be discharged from the opening 11together with the return water flow so that the sand is not accumulatedand the space inside the reef may be always maintained.

[0049] The ripraps at the coast side of the reef 2 dissipate the wavesthat cannot be captured by the slit 14 together with dissipating theenergy converted to flows from the waves at the upper portion of thereef. Because generation of the water flow from the coast to theoffshore is suppressed, a movement of the sand at the sea bottom becomessmall so that erosion of the beach may be prevented. In addition, aplurality of square holes 15 are formed through the bottom of the reef 2so that no great uplift force does affects a bottom face of the reef 2.

[0050] The embodiment in FIG. 4 shows the breakwater generatingstructure 1 formed by constructing the riprap mound 3 having the heightof 3 m, the length of 40 m, and the slope of 1:2 on the sea bottom andplacing the reef 2 made of pre-stressed concrete having the width of 10m, the height of 3 m, and the length of 15 m on the mound 3 at 10 m awayfrom the offshore end thereof.

[0051] The upper portion of the reef 2 is opened and the opening 11having the height of 40 cm is made on the lower end of the vertical wall10. The inner space of the reef is partitioned by the partition walls12. In the upper opening of the reef 2, the slit plates 13 aslant for 30degrees against the direction along which the waves propagate aredisposed with spacing each other between the partition walls 12 to formthe slit 14. A plurality of the square holes 15 are formed on the bottomof the reef 2.

[0052] The ripraps are laid at the coast side of the breakwatergenerating structure 1 and concrete blocks are laid on the surfacethereof so that the roughness becomes high, thereby absorbing energy ofthe waves that are not captured by the slit 14.

[0053] The concrete blocks are laid on the surface of the mound 3, forpreventing the scouring by a downforce element of the return water flowtoward the offshore through the opening 11. Preferably, above concreteblocks are laid for ½ of the lower reef length X₁ or more.

[0054] Waves rushing into the breakwater generating structure 1 areamplified by the mound 3 rising from the sea bottom, then the breakingwaves are generated by sudden decrease of the water depth at thevertical wall 10. Thereafter the generated breaking waves rush into theslit 14, and their energy is dissipated within the reef and the returnwater flow toward the opening 11 is also generated.

[0055] The return water flow toward the offshore through the opening 11also promotes the generation of the breaking waves, and at the sametime, the breaking wave point is generated on the slit so that thebreaking waves may accurately rush into the slit.

[0056] The embodiment shown in FIG. 5 is essentially the same as theembodiment shown in FIG. 4, however, the portion of 5 m at the offshoreside of the upper portion of the reef 2 is constructed as a closedportion 16 and the remaining portion of 10 m is constructed as an openedportion 17. On the upper opened portion of the upper surface of the reef2, the slit plates 13 aslant for 30 degrees with respect to thedirection along which the waves propagate are placed with spacing eachother between the partition panels to form the slant slits 14.

[0057] Waves rushing into the breakwater generating structure areamplified by the mount 3 rising from the sea bottom, the breaking wavesare generated by sudden decrease of the water depth at the vertical wall10 at the upper closed portion 16. By rushing into the slit 14, theenergy of the breaking water is lowered and the return water flow towardthe opening 11 is generated at the same time.

[0058] The return water flow toward the offshore through the opening 11and the return water flow toward the offshore from the closed portion 16on the upper portion of the reef cooperatively promote the generation ofthe breaking waves, and at the same time, the breaking wave point isgenerated on the slit so that the breaking waves may accurately rushinto the slit.

[0059] Preferably, the closed portion 16 may be set to be one-third (⅓)to one-half (½) of the length X₂ of the reef 2.

[0060] In the embodiment shown in FIGS. 6 and 7, the reef 2 isconstructed on a support 5 which is constructed by driving legs 4 suchas concrete piles or steel pipes into the sea bottom, and its principlefor breakwater generating is essentially the same as the breakwatergenerating structure described herein above. The support 5 correspondsto the mound and the incoming waves are amplified at the top of thesupport 5. The breaking waves is generated at the vertical wall 10 ofthe reef 2 so that the breaking waves may be caused to rush into theslit 14.

[0061] A height of a rear wall 18 constructed at the coast side of thereef 2 is set to be higher than the height of the vertical wall 10constructed at the offshore side, and the mounting positions of the slitplate 13 become increasingly higher toward the coast. Accordingly, thebreaking waves are certainly captured by the slit 14 to prevent thewaves from transferring to the coast side over the rear wall 18.

[0062] In the legged breakwater generating structure described above,the rear wall 18 need not always to be higher than the vertical wall andmay have the same height with the height of the vertical wall.Basically, the mound is simply replaced by the support with legs.

[0063] The legs 4 are generally constructed with concrete piles, and theconstruction period may be reduced by manufacturing the support 5 as asteel structured jacket, then the steel piles are driven into the jacketto fix the jacket.

[0064] The breakwater generating structure with legs has an advantagethat an influence to the environment may be minimized because thestructure is constructed without filling the sea area and is effectivewhen a water depth for the construction is deep or a slope of sea bottomis steep. Alternatively, constructing a seated type breakwatergenerating structure on a soft and unstable sea bottom ground may causesinking, the structure with legs described above, wherein the piles aredriven to the foundation rock, is preferably adopted in order to preventthe sinking.

[0065] In addition, the present invention may be applied to an offshorebreakwater as well as the breakwater generating structure.

[0066] As shown in a sectional view of FIG. 8 and a front view in FIG.9, the reef 2 is made of concrete and has the upper opening portion witha width of 10 m, a height of 3 m, a length of 20 m, and its offshoreside end is constructed as the vertical wall 10 with the opening 11. Theinner space of the reef 2 is partitioned by the walls. On the upperportion of the reef 2, the slit plate 13 being inclined 30 degrees withrespect to the wave propagation direction, is spaced between the wallsto form inclined slits 14. In the described embodiment, the reef 2 isconstructed as one unit and a plurality unit is placed on the mound 3along the seashore to form a desired length of the breakwater.

[0067] The reef 2 is placed such that the depth of the reef from thewater level is set to be 0.5 m or more. At the coast side of the reef 2,the ripraps are laid to prevent the reef from moving and blocks are laidat the front thereof. Through openings 16 are formed in the side wall ofthe reef 2 at the coast side with spacing as shown in FIG. 9. Thespacing and sizes of the through openings are determined with respect tothe replacement time of the sea water of the coast side water area.

[0068] A pipe is connected to each of the openings 20 to form a pathwhere the sea water flows, and mouths are disposed at the ends of thepipes to widen the diameter of the pipe to reduce the velocity of thesea water so that the sea bottom at the coast side may not be disturbed.

[0069] Waves from the offshore become the breaking waves, then rush intothe upper portion of the reef 2. The breaking waves pass through theslant slit 14 and then their energy is lowered inside the reef 2. Thebreaking waves that have been converted to the water flow are guided tothe rear portion of the breakwater from the through opening 16 formedthrough the side wall at the coast side to the through paths 19. Becausethe breaking waves including air flow through the through paths 19, thesea water contains sufficient dissolved oxygen.

[0070] Also the breaking waves create in the reef the return water flowtoward the opening 11 of the reef 2 and discharge the sand brought intothe reef 2 to the outside from the opening 11.

[0071] The ripraps at the coast side of the reef 2 dissipate the wavesthat are not captured by the slant slit 14 together with lowering theenergy of the flow converted from the waves on the upper portion of thereef.

[0072] The embodiment depicted in FIG. 10 shows the breakwatergenerating structure 1 which is constructed by providing the riprapmound 3 having a height of 3 m, a total length of 40 m, and a slope of1:2 on the sea bottom and placing the reef 2 made of concrete having awidth of 10 m, a height of 3 m, and a length of 15 m at 10 m from theoffshore side end of the mound 3.

[0073] Although the breakwater generating structure is essentially thesame as the embodiment described in FIG. 7, the portion of 5 m from theoffshore side on the upper portion of the reef 2 is constructed as theclosed portion 19 and the remaining portion of 10 m is left opened toprovide the open portion 17. On the upper opened portion of the reef 2,the slit plates 13 aslant for 30 degrees with respect to the directionof the propagation of the waves are positioned with spacing each otherto form the slant slit 14.

[0074] In addition, a plurality of square holes are formed on the bottomof the reef 2 to make the surface to which the uplift force affectssmall for preventing the reef 2 from floating.

[0075] The ripraps are laid at the coast side of the breakwatergenerating structure 1 and the foot protection blocks are placed on thesurface thereof to dissipate the waves that is not captured by the slit14 together with making the roughness against the water flow high.

[0076] The concrete blocks are laid at the front surface of the mound 3for preventing scouring by the downforce element of the return waterflow toward the offshore through the opening 11. Preferably, theconcretd blocks in the described embodiment may be laid for one-half (½)of the lower reef length or more.

[0077] The breakwater generating structure according to the presentinvention may provide an breakwater generating efficiency equal to ormore while providing smaller size than the size of conventionalbreakwater generating structures by disposing the slit behind thevertical wall and guiding the breaking waves generated by the verticalwall to the slit.

[0078] Accordingly, the breakwater generating structure may beconstructed with a low cost while enabling the shoaling beach berecovered on the steeply slant eroded beach, enhancing a stability ofthe beach and a purification ability of the sea water such that anabundant beach environment may be created.

[0079] Furthermore, because the water area with a high degree ofcalmness can be created between the breakwater generating structure andthe beach, a region appropriate for marine leisure may be provided.

[0080] Also, the slit and the entire breakwater generating structure ofthe present invention function as a gathering-place for fish byincreasing variations of creatures and the amount of the dissolvedoxygen increased by supplying oxygen with the jet effect of the breakingwaves, thereby providing a preferable condition for upbringing creaturesand increase of variations of creatures.

[0081] Furthermore, when the breakwater generating structure inaccordance with the present invention applies to a littoral nourishmentand an artificial beach on a gentle gradient beach, fine sands may besupplied by the improved breakwater generating effect and particlediameters of the sands on the beach may be made small so that acomfortable beach may be created.

[0082] By forming the slits at the portion into which the strong jet ofthe compound breaking waves rush, creation of splash and horizontalswirl are inhibited. The waves that have rushed into the slit create thereturn water flow toward the offshore through the opening in thevertical wall and shift the breaking wave point so that the breakingwaves may easily be captured by the slit. Accordingly, because the waveenergy is lowered and regeneration of the waves may be prevented,breakwater generating is effectively performed while enabling the almostequal breakwater generating effect even though said breakwatergenerating structure has more compact size than the conventionalbreakwater generating structure which utilizes the breaking waves.

[0083] The double reef structure decreases the transmission factor alongwith generation of the compound type breaking waves as well as thedecrease of the reflection factor. Accordingly, the breakwatergenerating structure in accordance with the present invention lowers theenergy of waves in high efficiency and realizes effective dissipation ofthe waves.

[0084] The breakwater generating structure in accordance with thepresent invention may guide the breaking wave bringing air into thereef, then sends the sea water containing sufficient oxygen into thebehind of the breakwater through the through path from the behindthereof to the bottom of the reef to improve the lean oxygen state ofthe sea water in the bottom layer behind the breakwater.

[0085] Accordingly, the seawater behind the structure is frequentlyreplaced to supply sufficient oxygen and the adverse influence to fishand shellfish swarmed in the sea area made calm by the submergedbreakwater is eliminated.

What we claim is;
 1. A breakwater generating structure comprising a reefhaving slits on an upper portion thereof and a vertical wall definingsaid reef at a offshore side, said vertical wall having at least oneopening at a lower end of said vertical wall.
 2. The breakwatergenerating structure according to the claim 1, wherein said slits areinclined with respect to the direction along which waves propagate. 3.The breakwater generating structure according to the claims 1 or 2,wherein said reef has a breaking wave generation section formed as aclosed upper portion which is defined between said vertical wall andsaid slits.
 4. The breakwater generating structure according to any oneof the claims form 1 to 3, wherein at least one hole is formed at thebottom of said reef.
 5. A breakwater generating structure comprising areef having slits on an upper portion hereof and a vertical walldefining said reef at a offshore side, said vertical wall having atleast one opening at a lower side of said vertical wall, said reef isplaced on a mound formed as two stages.
 6. The breakwater generatingstructure according to the claim 5, wherein said slits are inclined withrespect to the direction along which waves propagate.
 7. The breakwatergenerating structure according to the claims 5 or 6, wherein said reefhas a breaking wave generation section formed as a closed upper portiondefined between said vertical wall and said slits.
 8. The breakwatergenerating structure according to the claim 5, wherein through paths isprovided from said reef to a coast side of said breakwater generatingstructure, said reef being placed on said mound.
 9. The breakwatergenerating structure according to the claim 8, wherein an opening end ofsaid through path is provided to a base of said mound.
 10. Thebreakwater generating structure, wherein the breakwater generatingstructure of any one of the claims from 1 to 4 is placed on a supportcomprising a leg structure.
 11. The breakwater generating structureaccording to any one of the claims from 1 to 6, or the claim 10, whereina height of a coast side wall of said reef is higher than a height ofsaid vertical wall and positions of said slit are arranged to becomehigher toward said coast.